Semiconductor device with silicon nitride films having different oxygen densities

ABSTRACT

A semiconductor device of an embodiment includes a semiconductor layer formed of a III-V group nitride semiconductor, a first silicon nitride film formed on the semiconductor layer, a gate electrode formed on the first silicon nitride film, a source electrode and a drain electrode formed on the semiconductor layer such that the gate electrode is interposed between the source electrode and the drain electrode, and a second silicon nitride film formed between the source electrode and the gate electrode and between the drain electrode and the gate electrode and having an oxygen atom density lower than that of the first silicon nitride film.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority fromJapanese Patent Application No. 2012-048899, filed on Mar. 6, 2012, theentire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor deviceand a method for manufacturing the same.

BACKGROUND

Nitride-based semiconductor materials having high electric fieldstrength are highly expected to be applied to semiconductor devices forpower electronics, high frequency power semiconductors, or the like.However, it is also known that they suffer from a phenomenon calledcurrent collapse in which a drain current decreases significantly duringapplication of a stress such as a high voltage, which influences thecharacteristics of semiconductor devices. Moreover, reduction in a gateleakage current indeed needs to be achieved to increase a breakdownvoltage of a semiconductor device, and use of a gate insulation film isknown to be useful in achieving the purpose.

In order to realize a high performance nitride-based semiconductordevice, it is demanded to achieve a configuration which can take balancebetween suppression of the current collapse and reduction in the gateleakage current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view that illustrates a configuration of asemiconductor device of a first embodiment;

FIG. 2 is a cross-sectional view showing a step of a method formanufacturing the semiconductor device of the first embodiment;

FIG. 3 is a cross-sectional view showing another step of the method formanufacturing the semiconductor device of the first embodiment;

FIG. 4 is a cross-sectional view showing a further step of the methodfor manufacturing the semiconductor device of the first embodiment;

FIG. 5 is a cross-sectional view showing a yet further step of themethod for manufacturing the semiconductor device of the firstembodiment;

FIG. 6 is a cross-sectional view showing a yet further step of themethod for manufacturing the semiconductor device of the firstembodiment;

FIG. 7 is a diagram showing densities of oxygen and hydrogen atoms insilicon nitride films deposited with an electron cyclotron-resonance(ECR) plasma film deposition method and a plasma-enhancedchemical-vapor-deposition (PE-CVD) method, respectively;

FIG. 8 is a diagram showing a current collapse suppressing effect;

FIG. 9 is a cross-sectional view showing a configuration of asemiconductor device of a second embodiment;

FIG. 10 is a cross-sectional view showing a configuration of asemiconductor device of a third embodiment; and

FIG. 11 is a cross-sectional view showing a configuration of asemiconductor device of a fourth embodiment.

DETAILED DESCRIPTION

A semiconductor device of an embodiment includes a semiconductor layerformed of a III-V group nitride semiconductor, a first silicon nitridefilm formed on the semiconductor layer, a gate electrode formed on thefirst silicon nitride film, a source electrode and a drain electrodeformed on the semiconductor layer such that the gate electrode isinterposed between the source electrode and the drain electrode, and asecond silicon nitride film which is formed between the source electrodeand the gate electrode and between the drain electrode and the gateelectrode, and which has an oxygen atom density lower than that of thefirst silicon nitride film.

First Embodiment

A semiconductor device of the present embodiment includes asemiconductor layer formed of a III-V group nitride semiconductor, afirst silicon nitride film formed on the semiconductor layer, a gateelectrode formed on the first silicon nitride film, a source electrodeand a drain electrode formed on the semiconductor layer such that thegate electrode is interposed between the source electrode and the drainelectrode, and a second silicon nitride film which is formed between thesource electrode and the gate electrode and between the drain electrodeand the gate electrode and which has an oxygen atom density lower thanthat of the first silicon nitride film.

FIG. 1 is a cross-sectional view showing a configuration of asemiconductor device of the present embodiment. The semiconductor deviceis formed on a semiconductor layer 10 formed of an III-V group nitridesemiconductor. The semiconductor layer 10 has a structure in which abarrier layer 10 b formed of, for example, any of gallium nitride (GaN),aluminum nitride gallium (AlGaN), or indium nitride aluminum (InAlN), ora combination thereof is stacked on an operation layer 10 a formed ofGaN. A hetero junction interface is formed between the operation layer10 a and the barrier layer 10 b. For example, the operation layer 10 ais 1 μm in film thickness, and the barrier layer 10 b is 30 nm in filmthickness.

The present embodiment shows an example of ahigh-electron-mobility-transistor (HEMT) which is a field effecttransistor using a hetero junction between two semiconductor layers.However, the number of semiconductor layers is not limited to two, andthe configuration of the present embodiment may be applied tosemiconductor layers having various layered-structures. The HEMT thatuses a hetero junction like the present embodiment can reduceon-resistance because the channel mobility is high, and thus is suitablyused for semiconductor devices for power electronics. Moreover, the highchannel mobility is also suitable for high frequency operation.

A first silicon nitride film 12 is formed on the semiconductor layer 10.The first silicon nitride film 12 functions as a gate insulation film. Agate electrode 14 is formed on the first silicon nitride film 12. Thegate electrode 14 is, for example, a metal electrode. The metalelectrode is, for example, a nickel (Ni) electrode or a titanium (Ti)electrode.

Moreover, a source electrode 16 and a drain electrode 18 are provided onthe semiconductor layer 10, with the gate electrode 14 interposedbetween them. Each of the source electrode 16 and the drain electrode 18is separated from the gate electrode 14. The source electrode 16 and thedrain electrode 18 are, for example, metal electrodes. The metalelectrode is an electrode containing, for example, aluminum (Al) as amain component.

A second silicon nitride film 20 which has an oxygen atom density lowerthan that of the first silicon nitride film 12 is formed on thesemiconductor layer 10 between the source electrode 16 and the drainelectrode 18. The second silicon nitride film 20 is formed in contactwith the semiconductor layer 10. The second silicon nitride film 20functions as a surface passivation film (or a passivation film) whichprotects the surface of the semiconductor layer 10 between the gateelectrode 14 and the source electrode 16 and between the gate electrode14 and the drain electrode 18.

As described above, a problem with such a semiconductor device whichuses a nitride-based semiconductor is the current collapse phenomenon inwhich the drain current decreases significantly when a stress such as ahigh voltage is applied. In suppressing the current collapse phenomenon,it is an effective way to form a silicon nitride film on the surface ofthe nitride-based semiconductor. It is because such a configuration canreduce the interface state density in the interface between thesemiconductor and the insulation film. Such a configuration isadvantageous not only for reduction in the current collapse which can beachieved due to the reduction in the interface state density but alsofor high frequency operation.

Moreover, it is necessary to suppress the gate leakage current toachieve the high breakdown voltage. Suppression of leakage current canbe achieved by using an insulation film with a high dielectric breakdownelectric field. However, when the interface state density in theinterface between the gate insulation film and the nitride-basedsemiconductor is high, hysteresis in device characteristic occurs, whichcauses a threshold voltage to fluctuate and which leads to an unstablesemiconductor device operation.

Moreover, although increasing the film thickness of a gate insulationfilm is an effectively way to reduce the gate leakage current, such atechnique brings about a problem that, as the thickness of the gateinsulation film is increased, the distance between a channel layer and agate electrode is increased. As a result, the threshold voltage of thetransistor greatly shifts to the negative side, which is disadvantageousfor a switching element.

In the present embodiment, the silicon nitride film is used for both thegate insulation film and the surface passivation film. The first siliconnitride film 12 with a high oxygen atom density is used for the gateinsulation film. As a result, it is possible to suppress the gateleakage current without increasing the interface state density.

The second silicon nitride film 20 having an oxygen atom density lowerthan that of the first silicon nitride film 12 is used as the surfacepassivation film. As a result, it becomes possible to reduce the numberof electrons trapped in the surface passivation film or between thesurface passivation film and semiconductor layer 10.

Therefore, according to the semiconductor device of the presentembodiment, the configuration which can take balance between suppressionof the current collapse and reduction in the gate leakage current to canbe achieved.

Here, the oxygen atom density of the first silicon nitride film isdesirably 1×10²¹ atoms/cm³ or less. It is because the characteristicfluctuation is likely to occur, when the oxygen atom density is higherthan 1×10²¹ atoms/cm³ and the interface state density is increased, dueto an increased interface state density and an increase in the number ofelectrons trapped in the gate insulation film and between the gateinsulation film and the semiconductor 10.

Moreover, it is desirable that the oxygen atom density of the secondsilicon nitride film 20 is one digit or more lower than the oxygen atomdensity of the first silicon nitride film 12. It is because, under sucha condition, the current collapse can be sufficiently suppressed.

In addition, a hydrogen atom density of the first silicon nitride film12 is desirably one digit or more lower than a hydrogen atom density ofthe second silicon nitride film 20, and more desirably lower by twodigits or more. It is because, as the hydrogen atom density of the firstsilicon nitride film 12 is decreased, the gate leakage current can bereduced. It is desirable that the absolute value of the hydrogen atomdensity is 1×10²¹ atoms/cm³ or below.

As for the first silicon nitride film 12 functioning as the gateinsulation film, it is desirable to have a thick film thickness in termsof reduction in leakage current. However, as described above, as thethickness is increased, the threshold voltage becomes larger. Therefore,it is desirable that the film thickness is 50 nm or less when it isconsidered that the silicon nitride film is used for a switching elementfor the power electronics.

Moreover, when a high electrical field is applied to an element servingas a semiconductor for power electronics, the high electrical field isalso applied to a surface passivation film of the element when theelement is a horizontal element. For this reason, the second siliconnitride film 20 serving as the surface passivation film needs to have apredetermined thickness, and the thickness is desirably 100 nm or more.

FIGS. 2 to 6 are cross-sectional views of steps of a method formanufacturing the semiconductor device of the present embodiment.

First, as shown in FIG. 2, a semiconductor layer 10 as a nitridesemiconductor is prepared in which a barrier layer 10 b is stacked on anoperation layer 10 a.

Next, a second silicon nitride film 20 is formed on the semiconductorlayer 10 as shown in FIG. 3. It is desirable that the second siliconnitride film 20 is formed by using a PE-CVD method from a view pointthat a silicon nitride film with a relatively low oxygen atom densitycan be formed with such a method.

After that, a mask is formed by using a lithography technology, and thena portion of the second silicon nitride film 20 in which a gateinsulation film and a gate electrode are to be formed is removed byusing an RIE (Reactive Ion Etching) technology, etc.

Next, a first silicon nitride film 12 to be a gate insulation film isformed on the semiconductor layer 10 from which the second siliconnitride film 20 is removed and on the second silicon nitride film 20 asshown in FIG. 4.

One of various methods such as a PE-CVD method, a catalytic-CVD(Cat-CVD) method, and an ECR sputtering method may be used as a methodfor depositing the first silicon nitride film 12. Among these methods,it is desirable to use the ECR plasma deposition method, which is a kindof the ECR sputtering method from a view point that a silicon nitridefilm with a relatively high oxygen atom density and a relatively lowhydrogen atom density can be formed with the method. One of variousmethods such as a PE-CVD method, a Cat-CVD method, and an ECR sputteringmethod may be used as a method for depositing the first silicon nitridefilm 12.

In the ECR plasma deposition method, since a solid source such assilicon and an ECR plasma stream of oxygen or nitrogen are brought intodirect contact with each other, a silicon nitride film can be depositedwithout producing an intermediate product unlike the CVD. Moreover, asilicon nitride film containing a small content of hydrogen can beformed by using the ECR plasma deposition method. In addition, a siliconnitride film with good step coverage can be deposited.

As a method for adding oxygen in the ECR plasma deposition method, a gasprepared by adding a trace amount of gaseous oxygen to a nitrogen gasmay be used, and oxygen coming out of quartz parts used in manufacturingequipment may be used.

After that, a metal film to be a gate electrode 14 is formed on thefirst silicon nitride film 12, for example, by using a sputteringmethod.

Next, a mask is formed by using a lithography technique, and then themetal film and the first silicon nitride film 12 are patterned by usingan RIE technology or the like, so that the gate insulation film and thegate electrode 14 are formed as shown in FIG. 5.

Next, a mask based on a resist 30 is formed by using a lithographytechnology as shown in FIG. 6, and then a portion of the second siliconnitride film 20 in which a source electrode and a drain electrode are tobe formed is removed by using an RIE technology or the like. After that,a metal film, for example, an aluminum film is formed on the resist 30,and then the metal film is partially removed by a lift-off method. As aresult, the aluminum film remains only at portions in which the sourceand drain electrodes are to be formed.

The semiconductor device shown in FIG. 1 is manufactured through theabove-described manufacturing method. Consequentially, the secondsilicon nitride film 20 having an oxygen atom density lower than that ofthe first silicon nitride film 12 is formed on the semiconductor layer10, on both sides of the gate electrode 14.

FIG. 7 is a diagram showing an example of oxygen and hydrogen atomdensities in silicon nitride films deposited by using an ECR plasmadeposition method and a PE-CVD method. The vertical axis indicates atomdensities in the films evaluated with SIMS (Scannning Ion MassSpectrometry).

As shown in FIG. 7, a silicon nitride film having a relatively highoxygen atom density and a relatively low hydrogen atom density can bedeposited by using an ECR plasma deposition method. Moreover, thesilicon nitride film with a relatively low oxygen atom density can bedeposited by using a PE-CVD method.

FIG. 8 is a diagram showing a current collapse suppressing effect. Thecurrent collapse was evaluated using evaluation transistors in which aSchottky junction was formed between a gate electrode and asemiconductor layer. A silicon nitride film deposited by using an ECRplasma deposition method and a silicon nitride film deposited by using aPE-CVD method were used as a surface passivation film formed between asource electrode and a gate electrode and between a drain electrode anda gate electrode.

A gate voltage was changed in a state in which a constant voltage of 10V was applied between the source electrode and the drain electrode. Thecurrent collapse was evaluated by calculating a ratio between a currentvalue (I (DC)) measured by DC measurement and a current value (I(pulse)) measured by pulse measurement. The measurements were performeda plurality of times for a predetermined time, and consequently a changewith time was evaluated. In the drawing, when the value on the verticalaxis was 1.0, the current collapse will not be generated.

As clearly understood from FIG. 8, the current collapse phenomenon issuppressed when a silicon nitride film deposited by using a PE-CVDmethod is used as a surface passivation film.

According to the semiconductor device of the present embodiment and themethod for manufacturing the same, a configuration which can takebalance between suppression of the current collapse and reduction in thegate leakage current is achieved.

Second Embodiment

FIG. 9 is a cross-sectional view showing a configuration of asemiconductor device of the present embodiment. The present embodimentis the same as the first embodiment except that a first silicon nitridefilm 12 is not provided between a gate electrode 14 and a second siliconnitride film 20 serving as a surface passivation film. Therefore, thecontents which are a duplicate of the first embodiment will not bedescribed.

The same effect as the first embodiment can be achieved also by thepresent embodiment.

Third Embodiment

FIG. 10 is a cross-sectional view showing a configuration of asemiconductor device of the present embodiment. The present embodimentis the same as the first embodiment except that a first silicon nitridefilm 12 serving as a gate insulation film and a gate electrode 14 areformed in a trench provided in a barrier layer 10 b of a semiconductorlayer 10. Therefore, the contents which are a duplicate of the firstembodiment will not be described.

The same effect as the first embodiment can be achieved also by thepresent embodiment. In addition, by shortening the distance between thegate electrode 14 and the operation layer 10 a, it becomes easy to forma field effect transistor of a normally-off type.

Fourth Embodiment

FIG. 11 is a cross-sectional view showing a configuration of asemiconductor device of the present embodiment. The present embodimentis the same as the first embodiment except that a first silicon nitridefilm 12 serving as a gate insulation film is provided between asemiconductor layer 10 and a second silicon nitride film 20 serving as asurface passivation film. Therefore, the contents which are a duplicateof the first embodiment will not be described.

The embodiments have been described in connection with an example of afield effect transistor using a hetero junction, but the embodiments arenot limited thereto. The present embodiments may be applied to othertransistors using an III-V group nitride semiconductor.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, a semiconductor device and a method ofmanufacturing the same described herein may be embodied in a variety ofother forms; furthermore, various omissions, substitutions and changesin the form of the devices and methods described herein may be madewithout departing from the spirit of the inventions. The accompanyingclaims and their equivalents are intended to cover such forms ormodifications as would fall within the scope and spirit of theinventions.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor layer formed of a III-V group nitride semiconductor; afirst silicon nitride film formed on the semiconductor layer; a gateelectrode formed on the first silicon nitride film; a source electrodeand a drain electrode formed on the semiconductor layer, the gateelectrode is interposed between the source electrode and the drainelectrode; and a second silicon nitride film formed between the sourceelectrode and the gate electrode and between the drain electrode and thegate electrode, the second silicon nitride film has an oxygen atomdensity lower than that of the first silicon nitride film, wherein ahydrogen atom density of the first silicon nitride film is one digit ormore lower than a hydrogen atom density of the second silicon nitridefilm.
 2. The device according to claim 1, wherein the second siliconnitride film is formed in contact with the semiconductor layer.
 3. Thedevice according to claim 1, wherein an oxygen atom density of thesecond silicon nitride film is one digit or more lower than an oxygenatom density of the first silicon nitride film.
 4. The device accordingto claim 1, wherein the oxygen atom density of the first silicon nitridefilm is equal to or lower than 1×10²¹ atoms/cm³.
 5. The device accordingto claim 1, wherein the semiconductor layer has a stacked structureincluding an operation layer formed of GaN, and a barrier layer formedon the operation layer and formed of any of AlGaN, GaN, or InAlN, or acombination thereof.